Rockets, missiles and other similar airborne structures that move through the air at very high speeds are subject to harsh environmental conditions during use. The component structures must be designed to withstand high temperatures and pressures that are experienced as the structures move through the air. Surface temperatures of 1500° C. or greater may be encountered as a result of aerodynamic heating. When the structures are self-propelled and include engines that burn solid or liquid fuel, the engines components and adjacent components are subject to even greater temperatures as a result of the burning fuel. The components of such airborne structures must be fabricated of materials having sufficient strength to maintain their structural integrity during flight so as to not compromise the performance of the structure.
Metals and metal alloys have been utilized for their structural strength and ability to withstand elevated temperatures. These materials can be used to fabricate various components or portions of components to impart the desired properties to the component. For example, U.S. Pat. No. 6,314,720 discloses a protective coating for the combustion chamber of a liquid-fueled rocket engine that includes a liner fabricated of a variety of metallic materials. U.S. Pat. No. 6,442,931 discloses a combustion chamber that includes a steel or nickel alloy casing, a copper or copper alloy internal firewall, and an internal nickel coating. Weight restrictions and cost concerns for such airborne structures, however, limit the feasibility of using materials such as metals and metal alloys.
Polymer composites are another option for materials selection that presents various advantages. For example, the high stiffness of polymer composites provides a rigid structure. Further, polymer composites have low densities, so that components fabricated of these materials are of lighter weight and well within applicable weight limits.
Although use of polymer composites for airborne structures can provide many advantages, material properties of such composites may restrict the use of polymer composites alone in high temperature and/or high-pressure applications. Significantly, polymer composites are not fire resistant, and a thermally resistant material must be used in connection with polymer coating so that the polymer composite does not soften or ignite during use. Additionally, the strength of the polymer composite alone often is not sufficient to withstand typical forces associated with missile systems and the like.
With rocket motor casings, polymer composites have been used in combination with a separate thermally protective material to shield the polymer composites from exposure to the harsh conditions encountered by the components. Thermally protective materials are bonded to the airframe in secondary manufacturing operations. These post-bonded heat shields are often damaged during handling, and are rendered susceptible to debonding and moisture degradation over time. As a result, the damaged heat shields typically must be repaired frequently over the life of the component, which is not only inefficient but costly, as well.
Ceramics also have been used to fabricate components of airborne structures. U.S. Pat. No. 6,460,807 discloses carbon fiber-reinforced carbon, carbon fiber-reinforced silicon carbide, and silicon carbide fiber-reinforced silicon carbide components. Hybrid components formed by mechanical treatment of blanks and sub-segments also are disclosed. As with other mechanically formed components, a strong likelihood exists for the layers of such hybrid structures to delaminate during use as a result of the extreme temperature and pressure conditions, particularly in combustive environments.
There remains a need for an integrally fabricated composite structure to provide a material system having sufficient strength, fire resistance and durability characteristics to withstand use in airborne structures and high temperature and pressure applications. Such a composite structure would avoid a need for secondary manufacturing operations, such as bonding operations, to minimize maintenance costs associated with the repair of debonded and damaged components over the life cycle of the component.